This invention relates to a reinforced construction of load-bearing surface of the plunger in a solenoid.
The plunger in a solenoid is pulled by magnetic force toward the yoke of the solenoid. The pulled plunger moves instantaneously at a very high speed, and strikes against a load (the mating push-pin of a solenoid) with high impact. The load-bearing surface of the plunger is thus likely to be worn out and deformed. When a plunger is made of laminated silicon steel plates, the magnetic lines of force, and therefore the pulling force, are parallel with the laminated silicon steel plates. Although the silicon steel plates are assembled mechanically by means of caulking as a rigid member, each of them is thin, and weak in mechanical strength against a large impact applied parallel with the surface of the laminated plate. Thus, a solenoid including a laminated plunger is not durable for a long time. However, a solenoid may be required to ensure repeated operations of more than several hundred thousands or even several millions of times.
In order to prolong the life of a solenoid, a steel block, as a load-bearing member, is attached to the laminated plunger in a conventional solenoid. This steel block is embedded in a recess in the end surface of the laminated plunger, and is fixed therein by means of soldering. The plunger reinforced in this way requires many production operations, and the cost of manufacture of the solenoid becomes uneconomical. Further, if the steel block is shifted out of the plunger during the operation of the solenoid, or if the steel block is not in close contact with the laminated silicon steel plates, the magnetic resistance in the solenoid increases, thereby decreasing the reactance of the coil of the electromagnet and increasing the current in the coil. Thus, the coil then becomes heated to a high temperature. As an alternative method for reinforcing the laminated plunger, it has been proposed in the prior art to produce molten hard metal by means of electric welding and to pour it in the hollow of the laminated plunger. In this method, however, most of the laminated plunger becomes heated to a very high temperature, and the magnetic property of the laminated silicon steel plates, as well as the insulating quality of their oxide coating, is destroyed. Furthermore, blow holes often occur in the molten hard metal. These blow holes remain in the reinforcing member, provided by cooling the molten metal, and make the member weak in mechanical strength against impact. Thus, the percentage of faulty products, liable to give trouble in use, becomes extremely high.